JP7698632B2 - Silica, coating material and method for producing silica - Google Patents

Description

The present invention relates to silica, paint and a method for producing silica.

Silica produced by flame pyrolysis of chlorosilane is a fine silica with a specific surface area of about 50 m ² /g to 500 m ² /g, and is generally called fumed silica. This fumed silica is mainly used as a filler/reinforcement material or thickener for transparent resins, and as a powder fluidizing agent, and has excellent dispersibility. For this reason, fumed silica is often used as a filler for silicone rubber, a thickener for polyester resins, a toner fluidizing agent, etc.

However, when using fumed silica as a matting agent in paint, this good dispersibility becomes a disadvantage. That is, even if fumed silica has a weak dispersing power, it disperses in paint to a size less than the wavelength of visible light. For this reason, fumed silica cannot generally be used as it is as a matting agent in paint. Therefore, silica obtained by crushing and classifying wet silica (silica produced in a solvent such as water) with a large particle size is used as a matting agent for paint. However, even when silica produced from wet silica is used as a matting agent, it is difficult to say that sufficient matting performance is necessarily obtained.

On the other hand, a technique has been proposed for using fumed silica, which cannot be used as a matting agent as it is, as a matting agent. For example, Patent Document 1 proposes a technique for using silicic acid with an aerogel-like structure obtained by blending 5 to 50% by weight of water with fumed silica and drying the resulting powdery mixture as a matting agent. In addition, a technique has been proposed that employs an approach significantly different from that described in Patent Document 1. For example, Patent Document 2 proposes a technique for using texture-coated silica obtained by spraying water and a thermoplastic elastomer onto fumed silica while mixing in a mixing vessel, then pulverizing the mixture, and subsequently drying the mixture as a matting agent.

Special Publication No. 56-44012 Patent No. 4440114

The inventors further investigated the technology described in Patent Document 1, which is one of the technologies described in Patent Document 1 and 2, which have significantly different technical approaches. As a result, they confirmed that even if silica obtained using the technology described in Patent Document 1 is used as a matting agent for paint, sufficient matting properties cannot necessarily be obtained.

The present invention has been made in consideration of the above circumstances, and aims to provide silica that exhibits high matting properties and suppresses the occurrence of cloudiness when used as a matting agent in paint, paint using said silica, and a method for producing said silica.

The above object is achieved by the present invention.
The silica of the present invention is characterized in that it has an aggregated structure in which primary particles are aggregated, has a particle size ratio R represented by the following formula (1) of 4.3 to 5.2, has an absorbance of 0.6 or less for light with a wavelength of 700 nm in an aqueous dispersion having a concentration of 1.48% by mass, and has a particle density of 2.18 g/ ^cm3 or more as measured with a He pycnometer.
・Formula (1) R= ^L D50/ ^C D50
[In the formula (1), ^L D50 represents the volume-based cumulative 50% particle diameter (μm) of the silica measured based on a laser diffraction scattering method, and ^C D50 represents the volume-based cumulative 50% particle diameter (μm) of the silica measured based on a Coulter counter method.]

In one embodiment of the silica of the present invention, the volume-based cumulative 50% particle diameter ^LD50 is preferably 1.7 μm or more.

In other embodiments of the silica of the present invention, it is preferable that the bulk density is 35 g/L or more.

Another embodiment of the silica of the present invention is preferably used as a matting agent in paints.

Another embodiment of the silica of the present invention is preferably such that the paint containing the silica of the present invention is any paint selected from the group consisting of clear paints and black paints.

The coating material of the present invention is characterized in that it contains at least a matting agent and a resin, and the matting agent contains silica having a structure in which primary particles are aggregated, a particle size ratio R represented by the following formula (1) being 4.3 to 5.2, an absorbance of a 1.48% by mass aqueous dispersion of silica for light with a wavelength of 700 nm being 0.6 or less, and a particle density measured with a He pycnometer being 2.18 g/ ^cm3 or more.
・Formula (1) R= ^L D50/ ^C D50
[In the formula (1), ^L D50 represents the volume-based cumulative 50% particle diameter (μm) of the silica measured based on a laser diffraction scattering method, and ^C D50 represents the volume-based cumulative 50% particle diameter (μm) of the silica measured based on a Coulter counter method.]

One embodiment of the paint of the present invention is preferably any one selected from the group consisting of clear paints and black paints.

The method for producing silica of the present invention is characterized in that silica is produced through at least a basic aqueous solution addition step in which 4 to 13 parts by mass of a basic aqueous solution containing a basic substance at a concentration of 5N or more is added to 100 parts by mass of fumed silica, the absorbance of which for light with a wavelength of 700 nm in an aqueous dispersion of 1.48% by mass is 0.14 or less and the bulk density is 40 g/L to 110 g/L, and a drying step in which the wet mixture in which the basic aqueous solution has been added to the fumed silica is heated at a temperature equal to or higher than the boiling point of the basic substance to dry it.

In one embodiment of the method for producing silica of the present invention, it is preferable that the basic aqueous solution is ammonia water having a concentration of 5N or more.

Another embodiment of the method for producing silica of the present invention further includes a grinding step in which the silica obtained through the drying step is ground using a grinding device selected from the group consisting of a jet mill and a pin mill, and in the grinding step, it is preferable to carry out the grinding treatment until the volume-based cumulative 50% particle diameter ^C D50 of the silica after the grinding treatment, measured based on the Coulter Counter method, is 3.5 μm or less.

According to the present invention, it is possible to provide silica that exhibits high matting properties and suppresses the occurrence of cloudiness when used as a matting agent in paint, paint using said silica, and a method for producing said silica.

1. Silica The silica of the present embodiment has an aggregated structure in which primary particles are aggregated, and is characterized in that <1> the particle size ratio R represented by the following formula (1) is 4.3 to 5.2, <2> the absorbance of an aqueous dispersion having a concentration of 1.48% by mass for light with a wavelength of 700 nm (hereinafter, this absorbance may be referred to as τ700) is 0.6 or less, and <3> the particle density measured with a He pycnometer is 2.18 g/ ^cm3 or more.
・Formula (1) R= ^L D50/ ^C D50
[In formula (1), ^L D50 represents the volume-based cumulative 50% particle diameter (μm) of silica measured based on the laser diffraction scattering method, and ^C D50 represents the volume-based cumulative 50% particle diameter (μm) of silica measured based on the Coulter counter method.]

When the silica of this embodiment is used as a matting agent for paint, by satisfying any one of the conditions <1> to <3> above, it becomes easy to improve the matting properties and/or suppress the occurrence of cloudiness. Furthermore, when the conditions <1> to <3> are satisfied simultaneously, it is possible to achieve high matting properties and suppress the occurrence of cloudiness. Each of <1> particle size ratio R, <2> τ700, and <3> particle density will be described in detail below.

<1> Particle size ratio R
In the laser diffraction scattering method used to measure the particle diameter ^L D50 shown in formula (1), scattering occurs at the outermost surface of each particle to be measured. Therefore, the particle diameter measured by the laser diffraction scattering method is very close to the particle diameter of the particle observed by, for example, SEM (scanning electron microscope). In contrast, in the Coulter counter method used to measure the particle diameter ^C D50 shown in formula (1), the size of the particle is measured based on the change in electrical resistance when the particle passes through an aperture. Therefore, when the particle to be measured has a porous structure, the void portion in the particle is filled with an electrolyte (for example, Isoton II used to measure the particle diameter ^C D50 described later) used as a dispersion solvent used in the measurement. At this time, since an electric current flows in the void portion, the particle diameter ^C D50 of the particle having a porous structure will show a significantly smaller value than the particle diameter measured by the laser diffraction scattering method or SEM. Therefore, the particle diameter ratio R shown in formula (1) can be said to be an index for evaluating the degree of voids of the particle to be measured.

As described above, the silica of this embodiment has a particle size ratio R shown in formula (1) of 4.3 to 5.2, so that the primary particles are loosely bonded and have a bulky aggregate structure (a porous structure with a high porosity). Therefore, since the silica of this embodiment has a structure with large unevenness on the surface of each silica particle, diffuse reflection of light is likely to occur on the silica particle surface, making it easy to improve the matte properties of the paint. In addition, since the number of silica particles contained per unit weight is also larger, it is easy to achieve an excellent matte effect even if the amount of silica of this embodiment added to the paint is small. In addition, with the improvement in matte properties, it is also easy to suppress the clouding of the paint film. Furthermore, when a clear paint or black paint using the silica of this embodiment is applied to a substrate (dark substrate) with a dark-colored coating surface, the matte effect is exerted and a paint film with excellent jet blackness is easily formed.

In addition, from the viewpoint of further enhancing the effect of improving the matte property, the lower limit of the particle size ratio R is preferably 4.5 or more, and more preferably 4.7 or more. On the other hand, the larger the particle size ratio R, the more bulky the silica particles will have an aggregate structure, so if such an aggregate structure can be maintained as it is in the paint, further improvement in the matte property is expected. However, if the particle size ratio R is too large, the bulky aggregate structure is likely to collapse due to the shear force applied to the silica particles when stirring and mixing during paint preparation, resulting in deterioration of the matte property and easy occurrence of cloudiness. Therefore, the particle size ratio R needs to be 5.2 or less, and preferably 5.0 or less.

For reference, in silica particles having an aggregate structure in which primary particles are aggregated, the oil absorption is generally used as an index for evaluating the porosity and bulkiness of the silica particles. However, the oil absorption may include the amount of oil absorbed in the voids formed in each silica particle, as well as the amount of oil present in the gaps between the silica particles. Also, as mentioned above, the voids formed in the silica particles are closely related to the matte property, but the gaps between the silica particles are not related to the matte property. For this reason, the oil absorption does not exactly correspond to the size of the voids in each silica particle, and is a parameter that has a relatively lower correlation with the matte property compared to the particle size ratio R.

<2> τ700
In addition, in the silica of this embodiment, the absorbance (τ700) of the water dispersion with a concentration of 1.48% by mass for light with a wavelength of 700 nm is 0.6 or less. By making τ700 0.6 or less, the dispersibility of the silica particles in the paint is improved, and the effective area of the silica particle surface that diffusely reflects light can be made larger. Therefore, it is easy to improve the matte property, and it is also easy to suppress the clouding of the coating film with the improvement of the matte property. Furthermore, when a clear paint or black paint using the silica of this embodiment is applied to a substrate (dark substrate) with a dark colored coating surface, the matte effect is exerted, and the formation of a coating film with excellent jet blackness is also easy. In addition, when the silica of this embodiment is used in a paint containing a solvent, by making τ700 0.6 or less, it is easy to obtain a paint that is easy to apply to the substrate and does not drip after application. In addition, τ700 is preferably 0.5 or less, more preferably 0.4 or less. Further, the lower limit of τ700 is not particularly limited, but in practical terms, the lower limit of τ700 is preferably 0.25 or more, and more preferably 0.30 or more.

<3> Particle density In addition, the silica of this embodiment has a particle density of 2.18 g/ ^cm3 or more measured with a He pycnometer. This makes it easy to suppress the clouding of the coating film. Furthermore, the glossiness of the coating film is more or less reduced with the suppression of clouding, which makes it easy to improve the matte properties as a result. The reason why the suppression of clouding is made easier by making the particle density 2.18 g/cm3 or ^more is as follows.

First, the refractive index (theoretical refractive index) of pure amorphous silica is 1.46. In contrast, the refractive index of urethane resin, which is widely used as a resin component used in paints, is about 1.5. In addition, the refractive index of various resins for paints other than urethane resin is generally larger than 1.46. Here, if fine voids exist inside each primary particle constituting the aggregated structure of silica particles, the refractive index of the silica particles decreases in proportion to the presence of the voids. Therefore, when a coating film is formed using a coating film using such silica particles, the refractive index difference between the resin constituting the coating film and the silica particles becomes large, and the coating film becomes cloudy due to such a refractive index difference. Therefore, the refractive index of the silica particles is preferably as close as possible to the theoretical refractive index of 1.46, and for this purpose, it is better to have fewer voids inside each primary particle constituting the aggregated structure of the silica particles. Here, fewer voids inside the primary particles means that the particle density of the primary particles is high. Therefore, in the silica of this embodiment, the particle density is set to 2.18 g/cm ³ or more.

For reference, when particle density is measured using a He pycnometer, the measurement sample is made of silica particles compressed at high pressure to destroy the aggregate structure. Therefore, the particle density measured using a He pycnometer essentially corresponds to the density of the primary particles that make up the silica particles.

The particle density is preferably 2.185 g/cm ³ or more, and more preferably 2.19 g/cm ³ or more. The upper limit of the particle density is not particularly limited, but in practice, it is preferably 2.21 g/cm ³ or less, which is a value close to the true density of amorphous silica, and may be 2.205 g/cm ³ or less.

In addition, since the silica of this embodiment needs to have a size that does not transmit visible light at least in order to exhibit matte properties, its average particle size should be at least larger than the wavelength region of visible light (about 0.4 μm to 0.76 μm). The matte properties are exhibited by the silica particles partially embedded in the coating film surface, where the silica particles protrude from the coating film surface, scattering the light incident from the outside. That is, only the part of the silica particles that is slightly smaller than the part corresponding to the actual particle size (the part where the silica particles protrude from the coating film surface) contributes to light scattering. Therefore, taking this into consideration, in order to more reliably exhibit matte properties, the particle size of the silica of this embodiment is preferably slightly larger than the wavelength region of visible light, specifically, the particle diameter ^L D50 is preferably 1.7 μm or more, more preferably 3.0 μm or more, and even more preferably 5.0 μm or more. The upper limit of the particle diameter ^L D50 is not particularly limited, but in practical use, it is preferably 25.0 μm or less.

The pH of the silica of this embodiment is not particularly limited, but it usually has a pH value ranging from weakly acidic to weakly basic, and the pH value when the silica of this embodiment is dispersed in water and measured is in the range of about 5.5 to 9.5. The details of the pH measurement method will be described later.

The bulk density of the silica of this embodiment is not particularly limited, but the lower limit is preferably 35 g/L or more, more preferably more than 36 g/L, and even more preferably 43 g/L or more. On the other hand, the upper limit is not particularly limited, but in practice, 70 g/L or less is preferable, and 65 g/L or less is more preferable. By making the bulk density 35 g/L or more, when dispersing the matting agent made of the silica of this embodiment in a resin composition for paint, it is possible to prevent the silica from being unable to be uniformly dispersed, or to easily shorten the time required for the silica to be uniformly dispersed.

2. Paint The silica of this embodiment can be used for various purposes, but is particularly suitable for use as a matting agent for paint. In this case, the paint of this embodiment contains at least a matting agent (silica of this embodiment) and a resin. The paint of this embodiment may be any of a colored paint containing a coloring material such as a pigment or dye, a clear paint containing no coloring material (colorless and transparent), or a clear paint containing a small amount of coloring material (slightly colored) within a range that does not impair the transparency of the coating film. Furthermore, when the paint of this embodiment is liquid, the paint further contains a solvent such as water or an organic solvent. The paint of this embodiment may also contain various additives other than the silica of this embodiment used as a matting agent.

The paint of this embodiment can be used in the form of a solvent-based paint, an ultraviolet (UV) curing paint, a powder paint, etc., and specifically, can be used in the form of a water-based paint, an oil-based paint, a nitrocellulose paint, an alkyd resin paint, an amino alkyd paint, a vinyl resin paint, an acrylic resin paint, an epoxy resin paint, a polyester resin paint, a chlorinated rubber paint, etc. Among these, the paint of this embodiment is preferably a vinyl chloride paint or a urethane paint used as a paint for synthetic leather.

The resins constituting the paint can be any resin used in paints without any particular restrictions, and examples include one or more of the following: rosin, ester gum, pentalosin, coumarone-indene resin, phenolic resin, modified phenolic resin, maleic resin, alkyd resin, amino resin, vinyl resin, petroleum resin, epoxy resin, polyester resin, styrene resin, acrylic resin, silicone resin, rubber-based resin, chlorinated resin, urethane resin, polyamide resin, polyimide resin, fluorine resin, natural or synthetic lacquer, etc.

In addition, in UV-curable paints, high solid resins such as UV-curable acrylic resins, epoxy resins, vinyl urethane resins, acrylic urethane resins, polyester resins, etc. are usually used alone or in combination of two or more. In addition, in powder paints, thermoplastic resins such as polyamide, polyester, acrylic resin, olefin resin, cellulose derivatives, polyether, vinyl chloride resin, etc., as well as epoxy resins, epoxy/novolac resins, isocyanates, or epoxy-curable polyester resins, etc. are blended.

In addition, when the paint of this embodiment is a solvent-based paint, an organic solvent is used as the solvent. Examples of organic solvents include aromatic hydrocarbon solvents such as toluene and xylene; aliphatic hydrocarbon solvents such as n-heptane, n-hexane, and isopar; alicyclic hydrocarbon solvents such as cyclohexane; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; alcohol solvents such as ethanol, propanol, butanol, and diacetone alcohol; ether solvents such as tetrahydrofuran and dioxane; cellosolve solvents such as ethyl cellosolve and butyl cellosolve; ester solvents such as ethyl acetate and butyl acetate; and aprotic polar solvents such as dimethylformamide, dimethylacetamide, and dimethylsulfoxide.

When the silica of this embodiment is blended into a paint as a matting agent, the blending amount is appropriately set based on the various physical properties required of the paint, but typically, 5 to 33 parts by mass, and more preferably 10 to 30 parts by mass, per 100 parts by mass of solids contained in the paint is preferred. By blending an amount of 5 parts by mass or more, it becomes easier to achieve matting properties, and by blending an amount of 33 parts by mass or less, it becomes easier to ensure the strength of the coating film and suppress clouding.

As described above, the paint of this embodiment can be used as various types of paint, but is particularly suitable for use as a clear paint or black paint. When the paint of this embodiment is used as a clear paint or black paint to form a coating film on a substrate (dark substrate) with a dark-colored coating surface, a coating film with high jet black color can also be obtained.

The dark-colored substrate may be any material, as long as the coating surface is a dark-colored substrate such as black, navy blue, crimson, or dark green, but a substrate made of synthetic leather is suitable. When the paint of this embodiment is used as a clear coating, a small amount of black pigment such as carbon black or aniline black can be added to the paint of this embodiment as long as the transparency of the coating film is not impaired. In this case, the jet blackness of the coating film can be further increased.

When the dark-colored substrate is made of synthetic leather, the paint of this embodiment used as the clear paint or black paint is preferably a vinyl chloride paint or a urethane paint, as described above. Generally, aromatic urethane paints are relatively inexpensive and have high weather resistance, but aliphatic urethane paints are more commonly used in terms of appearance, such as jet blackness. However, by using a urethane paint containing an aromatic urethane resin to which silica has been added as a matting agent according to this embodiment, it is easy to obtain a coating film with a jet blackness similar to that of a conventional urethane paint containing an aliphatic urethane resin to which silica has been added as a matting agent.

3. Method for Producing Silica The silica of this embodiment is not particularly limited as long as it is produced through a process of aggregating raw material particles. The raw material particles correspond to the primary particles that form the aggregate structure in the silica of this embodiment. Here, fumed silica is usually used as the raw material particles. Fumed silica is produced by hydrolyzing a silica precursor such as a silane compound in a flame. Fumed silica is treated at an extremely high temperature during its production, and is therefore distinguished from wet silica that forms silica in an aqueous medium. Any type of fumed silica can be used as the fumed silica, but it is preferable to use fumed silica that has not been surface-treated with a surface treatment agent such as a hydrophobic treatment agent.

However, from the viewpoint of easily obtaining the silica of this embodiment, it is particularly preferable that the method of producing silica of this embodiment is a method of producing silica through at least a basic aqueous solution addition step of adding 4 to 13 parts by mass of a basic aqueous solution containing a basic substance at a concentration of 5N or more to 100 parts by mass of fumed silica having an absorbance of 0.14 or less for light with a wavelength of 700 nm in an aqueous dispersion of 1.48% by mass and a bulk density of 40 g/L to 110 g/L, and a drying step of heating and drying the wet mixture in which the basic aqueous solution is added to the fumed silica at a temperature equal to or higher than the boiling point of the basic substance. After the drying step, it is usually preferable to carry out a pulverization step or a classification step as appropriate in order to obtain silica having a desired particle size and particle size distribution. Details of each step are explained below.

In the basic aqueous solution addition process, fumed silica is used as the raw material particles, in which the absorbance (τ700) of a 1.48% by mass aqueous dispersion of the silica at a wavelength of 700 nm is 0.14 or less and the bulk density is 40 g/L to 110 g/L. By using fumed silica that satisfies the above conditions, the silica of this embodiment can be easily manufactured.

The particle density of the fumed silica used as the raw material particles (measured by a He pycnometer) is usually 2.20 g/cm ³ to 2.21 g/cm ^3. Therefore, it is easy to control the particle density of the silica (aggregate of primary particles) produced using fumed silica as the raw material particles to 2.18 g/cm ³ or more.

Furthermore, by setting the bulk density of the fumed silica to 40 g/L to 110 g/L, it becomes easier to control the particle size ratio R of the resulting silica to within the range of 4.3 to 5.2. The bulk density is preferably 50 g/L to 100 g/L. Furthermore, by setting the τ700 of the fumed silica to 0.14 or less, it becomes easier to control the τ700 of the resulting silica to 0.6 or less. It is preferable that τ700 is 0.13 or less, and although there is no particular lower limit, it is preferable that it is 0.06 or more in practical use.

In addition, most commercially available fumed silicas have a τ700 of 0.14 or less. Such silicas have a small particle size and good dispersibility in aqueous media, so the τ700 value is significantly smaller than that of the silica of this embodiment.

In addition, when using fumed silica with a bulk density of less than 40 g/L as raw material particles, a large-capacity processing device must be used when flocculating the raw material particles by adding a basic aqueous solution . In addition, the bulk density of the produced silica is also low. When using silica with a low bulk density as a matting agent, the silica is suspended on the resin composition during dispersion in the resin composition for paint, and dispersion in the resin composition is significantly hindered, and it takes time to disperse uniformly in the resin composition, or it may not be dispersed at all in the resin composition. Furthermore, by using fumed silica with a bulk density of less than 40 g/L as raw material particles, the cost of the production device increases, and the filling weight when packaging the produced silica is reduced, which also tends to increase the transportation cost.

Generally, fumed silica has an extremely low bulk density, so that when low-bulk-density fumed silica is treated with a basic aqueous solution, a large amount of the basic aqueous solution is required. If a large amount of basic aqueous solution is added to fumed silica, the silica produced will shrink significantly when the basic aqueous solution is evaporated and dried in the next drying step, making it difficult to obtain silica having a bulky aggregate structure. Therefore, from this viewpoint, it is preferable to set the bulk density to 40 g/L or more. If the bulk density of the obtained fumed silica is less than 40 g/L, the fumed silica may be compressed using a degassing press or the like to adjust the bulk density to within the range of 40 g/L to 110 g/L, and then used in the basic aqueous solution addition step.

The fumed silica used as the raw material particles usually has a specific surface area of about 190 m ² /g to 500 ^{m 2} /g, and particularly preferably 220 m ² /g to 400 m ² /g. On the other hand, the specific surface area of the silica of this embodiment tends to be smaller than that of the fumed silica used as the raw material particles, since the raw material particle surfaces are dissolved by the action of the added basic aqueous solution, and is usually 180 m ² /g to 350 m ² /g. Furthermore, the fumed silica used as the raw material particles preferably has a small amount of coarse particles, and particularly preferably has a sieve residue of 0.01% by weight or less when measured by the Mocca sieve method with an opening of 45 μm.

In the basic aqueous solution addition step, 4 to 13 parts by mass of a basic aqueous solution containing a basic substance at a concentration of 5N or more is added to 100 parts by mass of fumed silica having the above-mentioned τ700 of 0.14 or less and a bulk density of 40 g/L to 110 g/L. The preferred amount of the basic aqueous solution to be added is 4.5 to 8 parts by mass.

By setting the amount of the basic aqueous solution to 4 parts by mass or more, it becomes easy to uniformly treat the fumed silica used as the raw material particles, so that the silica of this embodiment can be stably obtained. In addition, it is also easy to control the particle size ratio R to 4.3 or more. Furthermore, by setting the amount of the basic aqueous solution to 13 parts by mass or less, it is possible to save the energy required for drying in the drying process, which is the next process, and/or shorten the drying time. Therefore, it is possible to suppress strong aggregation of the raw material particles treated with the basic aqueous solution, and as a result, it is possible to stably obtain the silica of this embodiment. Furthermore, empirically, the particle density of the silica of this embodiment tends to be slightly lower than the particle density of the fumed silica used as the raw material particles, and the decrease in particle density tends to be particularly noticeable when a large amount of a high-concentration basic aqueous solution is used. Therefore, from this perspective, it is preferable to set the amount of the basic aqueous solution to 13 parts by mass or less. Although the details of the reason why the particle density decreases are unknown, it is presumed that one of the reasons is that the primary particles of silica (aggregates) are made up of raw material particles whose structure has been slightly modified due to the surface dissolution, etc. of the fumed silica by the action of a base.

When a basic aqueous solution is added to the raw material particles, the surface dissolves due to the action of the base, and some of the dissolved surface bonds with other raw material particles to form aggregates. Because these aggregates are not very strong, silica with a bulky and loose aggregate structure is obtained.

In addition, when a basic aqueous solution is added to the raw material particles to wet the raw material particles with the basic aqueous solution, the heat treatment may be carried out at a temperature below the boiling point of water.

As the basic aqueous solution, a solution containing a basic substance at a concentration of 5N or more is used. By setting the concentration at 5N or more, the above-mentioned dissolution and formation of aggregates are sufficiently promoted, and as a result, the silica of this embodiment can be stably obtained. In addition, it becomes particularly easy to control τ700 to 0.6 or less. The concentration is preferably 7N or more, and more preferably 10N or more. On the other hand, the upper limit of the concentration is preferably 20N or less from a practical viewpoint.

As the basic substance, ammonia or water-soluble amines such as methylamine, dimethylamine, ethylenediamine, tetramethylammonium, and tetraethylammonium are preferred, and ammonia is particularly preferred, from the viewpoint of being easily removed from the surface of the aggregates of raw material particles by heat treatment in the drying process, which is the next process. For example, if the basic aqueous solution is 10% ammonia water by mass, the concentration of ammonia, which is a basic substance, is about 5.6N. Considering ease of availability and preparation, the basic aqueous solution is preferably ammonia water having a concentration of 5N or more, and in terms of concentration converted to ammonia content (mass%), ammonia water of 9.8% by mass or more is preferred, ammonia water of 10% by mass or more is more preferred, and ammonia water of 20% by mass or more is even more preferred.

When adding the basic aqueous solution, it is preferable to spray the basic aqueous solution while stirring the raw material particles in the container with a stirring blade or while gas is flowing, etc., so that the basic aqueous solution comes into uniform contact with the raw material particles. When spraying, it is preferable to add the basic aqueous solution using a one-fluid nozzle, two-fluid nozzle, or ultrasonic spray, which is easy to use. In this case, it is more preferable to select the average particle size of the spray liquid to be 100 μm or less. Furthermore, the basic aqueous solution may be supplied intermittently or continuously to the reactor container containing the raw material particles.

Considering that the surface of the raw material particles is dissolved and the raw material particles are appropriately coagulated, the reaction temperature must be within the range in which the basic aqueous solution is liquid, and is generally preferably about 15°C to 85°C. In addition, if the reaction time is too short, dissolution is difficult to occur, and if it is too long, coagulation tends to proceed too much, so it is preferable to carry out the reaction within the range of 0.4 hours to 3 hours. The reaction time here refers to the time from the time when the basic aqueous solution is added to the raw material particles to the time when heating is started to carry out the next step, the drying step. The pressure when carrying out the contact treatment between the basic aqueous solution and the raw material particles is not particularly limited, and can be appropriately selected from the range of negative pressure to pressure. In addition, the contact treatment may be carried out in a batch system or a continuous system.

In the basic aqueous solution addition step, after the contact treatment between the basic aqueous solution and the fumed silica used as the raw material particles is completed, a drying step is performed. In the drying step, the wet mixture in which the basic aqueous solution is added to the fumed silica is heated at a temperature equal to or higher than the boiling point of the basic substance to be dried. However, when the boiling point of the basic substance contained in the basic aqueous solution used is lower than the boiling point of water, it is necessary to heat at a temperature equal to or higher than the boiling point of water in the drying step. For example, when the basic substance is ammonia (boiling point at atmospheric pressure is about -33°C) and the drying step is performed under atmospheric pressure, the drying step is performed by heating to a temperature equal to or higher than the boiling point of water (100°C at atmospheric pressure). By performing the heat treatment in this manner, the basic substance can be quickly and sufficiently removed from the surface of the aggregate of the raw material particles. This also makes it possible to suppress the progression of excessive aggregation in the drying step, so that even when a basic aqueous solution addition step is carried out using a basic aqueous solution containing a high concentration of basic substance of 5N or more, it becomes easier to control the particle size ratio R of the obtained silica to within the ranges of 4.3 to 5.2, τ700 to 0.6 or less, and particle density to 2.18 g/ ^cm3 or more. As a result, it becomes easier to stably obtain the silica of this embodiment.

In addition, when the boiling point of the basic substance contained in the basic aqueous solution used is lower than the boiling point of water, when the drying process is performed under atmospheric pressure, the heating temperature may be 100°C or higher, but 150°C or higher is preferable. By setting the heating temperature to 100°C or higher, the drying time can be reduced and the basic substance can be sufficiently removed. In addition, when the drying process is performed under a pressure other than atmospheric pressure, the heating temperature may be equal to or higher than the boiling point of water under that pressure, and is preferably equal to or higher than the boiling point of water under that pressure + 50°C. In addition, the upper limit of the heating temperature during the drying process is not particularly limited, but considering the physical heat resistance of the heating device used in the drying process, it is preferable that it is 300°C or lower. In addition, during the drying process, it is preferable to raise the temperature to the desired heating temperature at a heating rate of 100°C/hr or less, and it is preferable to perform the drying process under an inert gas atmosphere by supplying an inert gas such as nitrogen gas during the drying process.

It is usually preferable to carry out a pulverization process on the silica obtained through the drying process (aggregates of primary particles corresponding to the raw material particles) in order to adjust the particle size and particle size distribution. As a pulverization device, it is preferable to use a pulverization device such as a jet mill or a pin mill that does not easily cause compression of the powder to be pulverized, and a jet mill is particularly preferable. In addition, when silica after drying is pulverized using such a pulverization device, a loose correlation is observed between the particle size ^C D50 and the particle size ratio R, and the particle size ratio R tends to increase as the particle size ^C D50 decreases.

In this case, if the pulverization treatment is performed until the particle diameter ^C D50 is 4.0 μm or less, it is easy to control the particle diameter ratio R to 4.3 or more. In order to more reliably control the particle diameter ratio R to 4.3 or more, the pulverization treatment is more preferably performed until the particle diameter ^C D50 at the end of the pulverization treatment is 3.5 μm or less, more preferably until it is 2.6 μm or less, and particularly preferably until it is 1.8 μm or less. On the other hand, from the practical viewpoint of avoiding a decrease in productivity due to a long pulverization treatment time, the pulverization treatment is preferably performed in a range in which the particle diameter ^C D50 at the end of the pulverization treatment is 0.8 μm or more, more preferably in a range in which it is 1.1 μm or more, and even more preferably in a range in which it is 1.2 μm or more. In addition, taking into consideration the circumstances of the manufacturing process as described above, the particle diameter ^C D50 of the silica of this embodiment is preferably 1.1 μm to 3.5 μm, more preferably 1.2 μm to 2.6 μm, and further preferably 1.2 μm to 1.8 μm.

In addition, a classification process may be carried out as necessary for the silica obtained through the drying process or the silica obtained through the pulverization process in order to remove coarse particles contained in the silica. In addition, a stirring and mixing process may be carried out in which the raw material particles are stirred and mixed using a Henschel mixer or the like prior to the basic aqueous solution addition process.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The methods for measuring various physical properties and characteristic values in the examples and comparative examples described below are as follows.

I. Evaluation of Various Physical Properties Various physical properties of the fumed silica used as raw material particles in each of the Examples and Comparative Examples, and the silica blended in the coating material were measured as follows.

1. Specific Surface Area The specific surface area was measured by a nitrogen adsorption BET single point method using a specific surface area measuring device (SA-1000) manufactured by Shibata Rikagaku Co., Ltd.

2. Absorbance (τ700) of a 1.48% by mass aqueous dispersion at a wavelength of 700 nm
The measurement of τ700 was performed by the measurement method disclosed in Journal of Ceramic Society of Japan 101[6], 707-712 (1993). Specifically, 0.3 g of powder sample and 20 ml of distilled water were filled into a glass sample vial (manufactured by AS ONE, capacity 30 ml, outer diameter approximately 28 mm). Next, the probe tip of an ultrasonic cell disrupter (BRANSON Digital Sonifier Model 250, probe: 1/4 inch microtip) was placed 10 mm below the water surface of the mixture of 0.3 g of powder sample and distilled water filled in the sample vial. In this state, ultrasonic stirring was performed under conditions of output 39% (30 W) and dispersion time 180 seconds, to obtain an aqueous dispersion (powder sample 0.3 g concentration: 1.48 mass%) in which 0.3 g of powder sample was finely dispersed in distilled water. Subsequently, the absorbance of the obtained aqueous dispersion at a wavelength of 700 nm was measured using a spectrophotometer (V-530, manufactured by JASCO Corporation). The measurement cell used for the absorbance measurement was a quartz cell with a ground glass side and an optical path length of 10 mm.

3. Bulk density (ρ)
The bulk density was measured by the following procedure. First, a resin graduated cylinder with a capacity of 1 L was placed on an electronic balance and then tared. Next, about 1 L of a powder sample was filled into the resin graduated cylinder and the weight M (g) was recorded. Next, the volume V (ml) was measured after tapping 30 times by hand with a tapping height (drop distance) of 10 cm, and the bulk density ρ was calculated based on the following formula (2).
・Formula (2) Bulk density ρ = 1000 x M/V (g/L)

4. Volume-based cumulative 50% diameter ( ^LD50 ) measured by laser diffraction scattering method
The particle diameter ^LD50 was measured using a laser diffraction scattering type particle size distribution analyzer (LA950V2, manufactured by Horiba, Ltd.). The internal settings of the LA950V2 were set to the following conditions: Circulation 5, Agitaion 7, and UltraSonic 4 minutes. In addition, 0.1 g of dried silica powder was directly placed in the device and the measurement was performed.

5. Volume-based cumulative 50% particle size ( ^C D50) measured by the Coulter counter method
The particle size ^C D50 was measured by the following procedure. First, a mixture of 50 g of methanol and 0.2 g of silica powder was dispersed for 3 minutes using an ultrasonic cleaner (B1510J-MT, manufactured by Emerson Japan Co., Ltd.) to prepare an alcohol dispersion. Next, the particle size of the silica particles dispersed in the alcohol dispersion was measured using a particle size distribution measuring device (TA II type, manufactured by Coulter Co., Ltd.) with an aperture tube of 50 μm. Isoton II was used as the electrolyte for the particle size distribution measuring device.

6. Particle density by He pycnometer The particle density was measured by the following procedure. First, a powder sample to be measured was filled in a cemented carbide press die (diameter 50 mm x height 75 mm), and then the powder sample to be measured was compression molded (uniaxial press) under a pressure of 15 tons using a press device (MH-15TON press (ram diameter 55 mm) manufactured by MASADASEISAKUSHO). After applying pressure for about 2 seconds, the compressed powder sample was removed from the die. Next, the compressed silica was dried in a vacuum dryer at a temperature of 200°C and a pressure of -0.095 PaG or less for 8 hours, and then cooled to room temperature under reduced pressure in the vacuum dryer to obtain a measurement sample.

The obtained measurement samples were measured using a dry automatic density meter (Shimadzu Corporation, AccuPyc1330 type) with a 10 ml sample insert and He gas at a pressure of 0.16 Pa. The measurement temperature of the density meter during density measurement was kept at 25°C by circulating constant temperature water.

7. pH
The pH was measured by the following procedure. First, 100 ml of degassed pure water was added to 5 g of the powder sample and stirred with a stirrer for 10 minutes to prepare a slurry for pH measurement. Next, the pH of this slurry was measured using a HORIBA PH meter F-52. Standard solutions of pH 4 and pH 9 were used to calibrate the pH meter.

8. Dibutyl phthalate (DBP) oil absorption The DBP oil absorption of the silica powder was measured using an oil absorption measuring device Model H5000 manufactured by Asahi Soken Co., Ltd. in accordance with JIS K6217-4.

II. Evaluation of Various Physical Properties and Characteristic Values of Paints and Coating Films In order to evaluate the various physical properties and characteristic values of paints and coating films, paints and coating films were prepared according to the following procedure.

<Preparation of Paint>
A paint (clear paint) was prepared by mixing 50 g of a paint resin composition (Dainichiseika Chemicals Co., Ltd., Leatheroid LU-1500 (aromatic urethane paint resin solids content 20%), 33.3 g of MEK (methyl ethyl ketone), 16.7 g of DMF (dimethylformamide), and 2.5 g of silica powder of each of the Examples and Comparative Examples, and dispersing the mixture in a homomixer at 8000 rpm (circumferential speed 11.7 m/S) for 6 minutes.

<Preparation of Coating Film>
The above paint was applied to a urethane synthetic leather with a black painted surface using a bar coater No. 14. After painting, the surface was dried at 60°C for 1 hour and then left at room temperature for 12 hours to obtain a urethane synthetic leather with a coating film formed on the painted surface. The L ^* value of the painted surface of the urethane synthetic leather used to form the coating film was 25.0, and the gloss value (gloss value at an incidence angle of 60 degrees) was 3.5%. The L ^* value and gloss value of the painted surface of the urethane synthetic leather were measured using the measurement method described below.

9. Grain Gauge Value The grain gauge value was measured using a 100 μm grain gauge for the paint in accordance with JIS K 5600-2-5.

10. Viscosity and TI
The viscosity of the paint was measured at 25° C., 60 rpm and 6 rpm using a BL type rotational viscometer after leaving the paint in a thermostatic water bath at 25° C. for 2 hours. The viscosity measured at 60 rpm is shown in Tables 3 and 4. The value obtained by dividing the viscosity at 6 rpm by the viscosity at 60 rpm was calculated as the TI (thixotropy index).

11. L ^* Value The L ^* value was measured on the surface of the urethane synthetic leather on which the coating was formed, using a spectrophotometer (CM-5 model, manufactured by Konica Minolta, Inc.). The L ^* a ^* b ^* (CIE1976) color system was used, and the L ^* value was measured using SCI (value including regular reflected light) with a measurement diameter of 8 mm. This L ^* value is an index of jet blackness.

12. Gloss Value The gloss value was measured for the surface of the urethane synthetic leather on which the coating film was formed, in accordance with JIS Z 8741. A gloss meter (IQ3 model, manufactured by RHOPINT) was used for the measurement, and the gloss (gloss value) was evaluated at an incidence angle of 60 degrees.

13. Visual Evaluation The surface of the urethane synthetic leather on which the coating film was formed was visually observed and evaluated. The evaluation criteria are as follows:
A: The jet black color is superior to the case of the following B rating.
B: No white areas are observed throughout the entire surface, and the surface has a sufficiently matte finish and jet black color.
C: Partial clouding is observed.
D: Overall turbidity is observed.

Example 1
Fumed silica with a specific surface area of 300 m ² /g, τ700 of 0.084, bulk density of 75 g/L, and particle density of 2.209 g/cm ³ was used as the raw material particles. 5 kg of the raw material particles were charged into a Henschel type mixer with an internal volume of 300 L and stirred and mixed, and then nitrogen gas was introduced into the mixer to replace the atmospheric gas in the mixer with nitrogen gas. Next, with the temperature inside the mixer heated to 80° C., 250 ml of ammonia water with a concentration of 25% by mass was supplied into the mixer at a flow rate of 500 ml/hr using a one-fluid nozzle to obtain a wet mixture in which the raw material particles were moistened with ammonia water. The fumed silica used as the raw material particles in Example 1 was not subjected to any surface treatment. This point is the same for the fumed silica used as the raw material particles in other Examples and Comparative Examples.

Next, the wet mixture was stirred and nitrogen was supplied into the mixer at 40 L/hr while the temperature was raised to 180° C. at 100° C./hr. The mixer was then held at 180° C. for 1 hour to obtain a dried silica powder. The dried silica powder was then pulverized using a jet mill (STJ315, manufactured by Seishin Enterprise Co., Ltd.) and adjusted to a particle size ^C D50 of 1.4 μm to obtain the silica powder of Example 1. In the following examples and comparative examples, unless otherwise specified, the jet mill used was STJ315, manufactured by Seishin Enterprise Co., Ltd.

Example 2
A silica powder of Example 2 was obtained by carrying out the same process as in Example 1, except that in the pulverization step, pulverization was carried out using a jet mill so that the particle size ^C D50 became 2.6 μm.

Example 3
The silica powder of Example ^{3 was obtained by carrying out the same process as in Example 1} , except that fumed silica having a specific surface area of 300 ^m2 /g, τ700 of 0.084, bulk density of 61 g/L, and particle density of 2.209 g/cm3 was used as the raw material particles, and 340 ml of ammonia water having a concentration of 18% by mass was added as the basic aqueous solution to the raw material particles.

Example 4
The silica powder of Example 4 was obtained by carrying out the same process as in Example ¹ , except that fumed silica having a specific surface area of 250 ^m2 /g, τ700 of 0.114, bulk density of 85 g/L, and particle density of 2.205 g/cm3 was used as the raw material particles, and 223 ml of ammonia water having a concentration of 25% by mass was added as the basic aqueous solution to the raw material particles.

Example 5
The basic aqueous solution adding step and the drying step were carried out in the same manner as in Example 1, except that the concentration of the ammonia water used as the basic aqueous solution added to the raw material particles was 9.8 mass %, and the amount of ammonia water added to the raw material particles was 600 ml. Thereafter, the silica after the drying treatment was pulverized by a jet mill so that the particle size ^C D50 was 1.3 μm, thereby obtaining a silica powder of Example 5.

Example 6
The basic aqueous solution addition step and drying step were carried out in the same manner as in Example 1, except that fumed silica with a specific surface area of 300 ^m2 /g, τ700 of 0.084, bulk density of 100 g/L, and particle density of 2.209 g/ ^cm3 was used as the raw material particles. The silica after the drying treatment was then pulverized by a jet mill so that the particle size ^C D50 was 1.1 μm, thereby obtaining silica powder of Example 6.

(Example 7)
The basic aqueous solution addition step and drying step were carried out in the same manner as in Example 1, except that fumed silica with a specific surface area of 380 ^m2 /g, τ700 of 0.067, bulk density of 55 g/L, and particle density of 2.210 g/ ^cm3 was used as the raw material particles. The silica after the drying treatment was then pulverized by a jet mill so that the particle size ^C D50 was 1.2 μm, thereby obtaining a silica powder of Example 7.

(Example 8)
The same process as in Example 1 was carried out except that the dried silica was pulverized by a jet mill so that the particle size ^C D50 was 3.5 μm, thereby obtaining a silica powder of Example 8.

(Example 9)
The raw material particles were fumed silica having a specific surface area of 225 ^m2 /g, τ700 of 0.128, bulk density of 110 g/L, and particle density of 2.203 g/ ^cm3 , and the basic aqueous solution adding step and drying step were carried out in the same manner as in Example 1, except that 500 ml of ammonia water having a concentration of 14% by mass was added to the raw material particles as the basic aqueous solution. Thereafter, the silica after the drying treatment was pulverized by a jet mill so that the particle diameter ^C D50 was 3.3 μm, thereby obtaining a silica powder of Example 9.

(Example 10)
The raw material particles were fumed silica having a specific surface area of 220 ^m2 /g, τ700 of 0.130, bulk density of 40 g/L, and particle density of 2.203 g/ ^cm3 , and the basic aqueous solution adding step and drying step were carried out in the same manner as in Example 1, except that 650 ml of ammonia water having a concentration of 10% by mass was added to the raw material particles as the basic aqueous solution. Thereafter, the silica after the drying treatment was pulverized by a jet mill so that the particle diameter ^C D50 was 3.0 μm, thereby obtaining a silica powder of Example 10.

Example 11
The same process as in Example 1 was carried out, except that the temperature in the mixer during and after the nitrogen replacement was kept at room temperature (20° C.), to obtain a silica powder of Example 11.

(Comparative Example 1)
The silica powder of Comparative Example 1 was obtained by carrying out the same process as in Example 1, except that the concentration of the ammonia water used as the basic aqueous solution was 2 mass % and the amount of ammonia water added to the raw material particles was 400 ml.

(Comparative Example 2)
The raw material particles were fumed silica with a specific surface area of 300 ^m2 /g, τ700 of 0.084, bulk density of 25 g/L, and particle density of 2.209 g/ ^cm3 , and the basic aqueous solution adding step and drying step were carried out in the same manner as in Example 1, except that 1000 ml of ammonia water with a concentration of 0.15 mass% was added to the raw material particles as the basic aqueous solution. Thereafter, the dried silica was pulverized by a jet mill so that the particle diameter ^C D50 was 2.2 μm, thereby obtaining a silica powder of Comparative Example 2.

(Comparative Example 3)
The silica powder of Comparative Example 3 was obtained by carrying out the same process as in Example 1, except that fumed silica with a specific surface area of 200 ^m2 /g, τ700 of 0.157, bulk density of 75 g/L, and particle density of 2.200 g/ ^cm3 was used as the raw material particles.

(Comparative Example 4)
The basic aqueous solution adding step and drying step were carried out in the same manner as in Example 1, except that 1500 ml of a basic aqueous solution (pH 10.8) prepared by diluting sodium silicate No. 3 defined in JIS K 1408 to a concentration of 1% by mass was added to the raw material particles as the basic aqueous solution. Thereafter, the silica after the drying treatment was pulverized by a jet mill so that the particle size ^C D50 was 2.3 μm, thereby obtaining a silica powder of Comparative Example 4.

(Comparative Example 5)
As the silica powder of Comparative Example 5, a commercially available wet-ground silica product (Tokuyama Corp., Finesil E-50, specific surface area 200 m ² /g) was used as it was.

(Comparative Example 6)
As the silica powder of Comparative Example 6, commercially available gel-process silica (Beijing Aerospace Satelite Co., Ltd., gel-process silica SD-450) was used as it was.

(Comparative Example 7)
Fumed silica with a specific surface area of 297 m ² /g, τ700 of 0.084, bulk density of 27 g/L, and particle density of 2.208 g/cm ³ was used as the raw material particles. 5 kg of the raw material particles were charged into a Henschel type mixer with an internal volume of 300 L and stirred and mixed, and then nitrogen gas was introduced into the mixer to replace the atmospheric gas in the mixer with nitrogen gas. Next, while maintaining the temperature in the mixer at room temperature (25° C.), 750 ml of ammonia water (pH=10.2) with a concentration of 10 ppm was supplied into the mixer using a one-fluid nozzle at a flow rate of 500 ml/hr to obtain a wet mixture in which the raw material particles were wetted with ammonia water.

Next, while continuing to stir the wet mixture, the temperature was raised to 180°C at 100°C/hr while supplying nitrogen into the mixer at 40 L/hr. The temperature inside the mixer was then held at 180°C for 1 hour to obtain the dried silica powder of Comparative Example 7.

(Comparative Example 8)
The silica powder of Comparative Example 7 was pulverized by a jet mill so that the particle size ^C D50 became 2.6 μm, thereby obtaining the silica powder of Comparative Example 8.

(Comparative Example 9)
The silica powder of Comparative Example 7 was pulverized by a jet mill so that the particle size ^C D50 became 1.1 μm, thereby obtaining a silica powder of Comparative Example 9.

(Comparative Example 10)
The basic aqueous solution addition step was carried out in the same manner as in Comparative Example 7 to obtain a wet mixture in which the raw material particles were wetted with ammonia water. The wet mixture was then pulverized using a single track jet mill (FS4 type, manufactured by Seishin Enterprise Co., Ltd.). The pulverized material obtained was then placed in a regular tray dryer and dried at a temperature of 120° C. until the moisture content in the pulverized material reached 3%, thereby obtaining a silica powder of Comparative Example 10.

(Comparative Example 11)
A wet mixture was obtained by wetting the raw material particles with ammonia water by carrying out the basic aqueous solution addition step in the same manner as in Comparative Example 7. The wet mixture was then pulverized using a pinned disk mill free pulverizer (M-3 type, manufactured by Nara Machinery Works, Ltd.). The pulverized material obtained was then placed in a 1 L wide-mouth metal bottle and dried at a temperature of 55°C for 20 minutes, and then placed in a regular tray dryer and dried at a temperature of 120°C until the moisture content in the pulverized material reached 3%, thereby obtaining a silica powder of Comparative Example 11.

(Comparative Example 12)
The silica powder of Comparative Example 11 was again pulverized using a pinned disc mill free pulverizer (M-3 type, manufactured by Nara Machinery Works, Ltd.) to obtain a silica powder of Comparative Example 12.

(Comparative Example 13)
Fumed silica with a specific surface area of 297 m ² /g, τ700 of 0.084, bulk density of 54 g/L, and particle density of 2.208 g/cm ³ was used as the raw material particles. 5 kg of the raw material particles were charged into a Henschel type mixer with an internal volume of 300 L and stirred and mixed, and then nitrogen gas was introduced into the mixer to replace the atmospheric gas in the mixer with nitrogen gas. Next, with the temperature inside the mixer heated to 70° C., 3000 ml of a sodium silicate aqueous solution (pH 10.8) in which sodium silicate No. 3 specified in JIS K 1408 was diluted to a concentration of 1 mass % was supplied into the mixer, thereby obtaining a wet mixture in which the raw material particles were wetted with the sodium silicate aqueous solution.

The wet mixture was then pulverized using a single track jet mill (FS4 model, manufactured by Seishin Enterprise Co., Ltd.). The pulverized material was then placed in a regular tray dryer and dried at 127°C until the moisture content of the pulverized material reached 4.2%, to obtain the silica powder of Comparative Example 13.

(Comparative Example 14)
Fumed silica having the same specific surface area, particle density, and τ700 as the fumed silica used in Example 1 except that the bulk density was 25 g/L was used as the raw material particles, and the basic aqueous solution addition step and drying step were carried out in the same manner as in Example 1 except that the raw material particles used were different. Thereafter, the silica after the drying treatment was pulverized by a jet mill so that the particle diameter ^C D50 was 1.1 μm, thereby obtaining a silica powder of Comparative Example 14.

(Comparative Example 15)
A fumed silica having the same specific surface area, particle density and τ700 as the fumed silica used in Example 1 except that the bulk density was 120 g/L was used as the raw material particles, and a process similar to that of Example 1 was carried out except that the raw material particles used were different, to obtain a silica powder of Comparative Example 15.

(Comparative Example 16)
Except for changing the amount of ammonia water added to the raw material particles to 780 ml, the basic aqueous solution adding step and the drying step were carried out in the same manner as in Example 1 to obtain a dried silica powder. Next, the dried silica was pulverized by a jet mill and adjusted to have a particle size ^C D50 of 1.4 μm to obtain a silica powder of Comparative Example 16.

(Comparative Example 17)
Except for changing the amount of ammonia water added to the raw material particles to 165 ml, the basic aqueous solution adding step and the drying step were carried out in the same manner as in Example 1 to obtain a dried silica powder. Next, the dried silica was pulverized by a jet mill and adjusted to have a particle size ^C D50 of 1.4 μm to obtain a silica powder of Comparative Example 17.

The manufacturing conditions of the silica powder of each Example and Comparative Example are shown in Tables 1 and 2. In addition, the physical properties of the paints prepared using the silica powder of each Example and Comparative Example, and the evaluation results of the coating films formed using these paints are shown in Tables 3 and 4.

<Evaluation of dispersibility in resin compositions for paints>
The silica powders of Examples 6 and 10 and the silica powder of Comparative Example 14 were evaluated for dispersibility in a resin composition (a resin composition containing an aliphatic urethane resin as a main component) used in preparing a coating material by the following procedure.

First, 3.75 g of silica powder was added to the top of 100 g of resin composition (BLLK-2000 (aliphatic urethane paint; solids content 15%, solvent content consisting of toluene/IPA (isopropyl alcohol) = 9/1 (mass ratio): 85%) manufactured by Shinran Technology Co., Ltd.) filled in a container. Next, the mixture was stirred at 2500 rpm (circumferential speed 5.2 m/S) using a disper blade mixer with an outer diameter of 40 mm. Stirring was continued until the silica powder was completely and uniformly dispersed throughout the resin composition. Whether or not the silica powder was completely and uniformly dispersed throughout the resin composition was confirmed visually. The time from the start of stirring to the end of stirring was then measured. As a result, it was confirmed that the time until the silica powder was completely and uniformly dispersed throughout the resin composition was 4 minutes for the silica powder of Example 6, 5.3 minutes for the silica powder of Example 10, and 8 minutes for the silica powder of Comparative Example 14.

Claims (10)

It has an aggregated structure in which primary particles are aggregated,
The particle size ratio R shown in the following formula (1) is 4.3 to 5.2,
The absorbance of an aqueous dispersion of the compound having a concentration of 1.48% by mass for light having a wavelength of 700 nm is 0.6 or less, and
Silica characterized in that the particle density measured by a He pycnometer is 2.18 g/ ^cm3 or more.
・Formula (1) R= ^L D50/ ^C D50
[In the formula (1), ^L D50 represents the volume-based cumulative 50% particle diameter (μm) of the silica measured based on a laser diffraction scattering method, and ^C D50 represents the volume-based cumulative 50% particle diameter (μm) of the silica measured based on a Coulter counter method.] 2. The silica according to claim 1, wherein the volume-based cumulative 50% particle diameter ^L D50 is 1.7 μm or more. Silica according to any one of claims 1 and 2, characterized in that it has a bulk density of 35 g/L or more. The silica according to any one of claims 1 to 3, which is used as a matting agent for paints. The silica according to claim 4, characterized in that the paint is any paint selected from the group consisting of clear paints and black paints. Contains at least a matting agent and a resin,
The matting agent contains silica having a structure in which primary particles are aggregated, a particle size ratio R represented by the following formula (1) being 4.3 to 5.2, an absorbance of a 1.48% by mass aqueous dispersion of silica for light having a wavelength of 700 nm being 0.6 or less, and a particle density measured by a He pycnometer being 2.18 g/cm3 or ^more .
・Formula (1) R= ^L D50/ ^C D50
[In the formula (1), ^L D50 represents the volume-based cumulative 50% particle diameter (μm) of the silica measured based on a laser diffraction scattering method, and ^C D50 represents the volume-based cumulative 50% particle diameter (μm) of the silica measured based on a Coulter counter method.] The paint according to claim 6, which is any one selected from the group consisting of clear paints and black paints. a basic aqueous solution adding step of preparing a wet mixture by adding 4 to 13 parts by mass of a basic aqueous solution containing a basic substance at a concentration of 5N or more to 100 parts by mass of fumed silica having an absorbance of 0.14 or less at a wavelength of 700 nm in an aqueous dispersion having a concentration of 1.48% by mass and a bulk density of 40 g/L to 110 g/L;
and a drying step of heating and drying the wet mixture at a temperature equal to or higher than the boiling point of the basic substance to produce silica. The method for producing silica according to claim 8, characterized in that the basic aqueous solution is ammonia water having a concentration of 5N or more. The method further comprises a grinding step of grinding the silica obtained through the drying step using a grinding device selected from the group consisting of a jet mill and a pin mill,
10. The method for producing silica according to claim 8 or 9, characterized in that in the pulverization step, the pulverization is carried out until the volume-based cumulative 50% particle diameter ^C D50 of the silica after the pulverization treatment, as measured by a Coulter Counter method, becomes 3.5 μm or less.